Off-road machines for felling trees and for handling felled trees ("feller bunchers") have achieved wide acceptance in the forest industry. Such machines typically have a crawler type undercarriage for mobility over uneven terrain, a superstructure attached to the undercarriage and an extendible boom attached to the superstructure. The superstructure typically houses an engine and hydraulic pump system to operate the machine's moving parts and generally comprises a cab from which the machine operator controls the machine. Accessories such as grapples, tree falling heads, cut off saws, or delimbing devices are typically mounted to the boom. The superstructure is typically mounted to the undercarriage on a pivot so that the superstructure and boom can be swung about a vertical axis.
Tree harvesting machines of the type described above are often used off-road on uneven terrain. Therefore, after the vehicle has been driven to a desired location to do work the undercarriage of the vehicle may not be horizontal and the axis of rotation of the superstructure may not be vertical. It is highly desirable, however, for the comfort of the operator and for the most efficient functioning of the machine that the superstructure of the machine be horizontal while the machine is working. Furthermore, in general, the trees which the machine harvests grow essentially vertically. When the machine is used to handle standing trees, as opposed to previously cut trees, and the superstructure is maintained horizontal while the machine is working then a boom-mounted accessory can be designed to work on a vertical tree without the need for an additional mechanism to tilt the accessory to align with the tree. Accordingly, it has been found to be useful to mount a tilting mechanism between the superstructure of the machine and the undercarriage. The operator of the machine can adjust the tilting mechanism to keep the superstructure horizontal even when the undercarriage of the machine is inclined at a significant angle to the horizontal.
Early tilting mechanisms only provided for rotation about a single axis. These tilting mechanisms are called "two-way" tilting mechanisms because they allow the angle of tilt to be adjusted in two directions (e.g. clockwise and anti-clockwise) about the single axis. To maintain the superstructure of a machine equipped with a two-way tilting mechanism horizontal the operator of the machine must first orient the machine with the tilting axis perpendicular to the fall line of the ground under the machine's undercarriage. A two-way tilting mechanism cannot generally maintain the superstructure of a machine horizontal when the tilting axis is not horizontal.
So called "four-way" tilting mechanisms allow the superstructure of a machine to be tilted back and forth about each one of a pair of mutually perpendicular axes. A four-way tilting mechanism is therefore capable of maintaining the superstructure of a machine level, within its limits of motion, independently of the direction of inclination of the undercarriage.
Currently available four-way tilting mechanisms have several disadvantages. Firstly, they can be complicated and expensive to manufacture; secondly, they can suffer from not being rugged enough to handle the forces typically exerted on off-road machinery; thirdly, most prior art four-way tilting mechanisms can not tilt equally in all directions. Most prior art four-way tilting mechanisms can tilt through a larger angular range about one axis than about a second axis. Operating off-road machinery equipped with such levelling mechanisms can therefore require a machine operator to continuously adjust the orientation of the machine's undercarriage when the machine is working on ground where a high degree of tilt is required to maintain the machine's superstructure level.
There is a need for a simple, robust tilting mechanism that avoids disadvantages of prior 2-way and 4-way levelling mechanisms.